In recent years, there has been development of organic semiconducting (OSC) materials in order to produce more versatile, lower cost electronic devices. Such materials find application in a wide range of devices or apparatus, including organic field effect transistors (OFETs), organic light emitting diodes (OLEDs), photodetectors, organic photovoltaic (OPV) cells, sensors, memory elements and logic circuits to name just a few. The organic semiconducting materials are typically present in the electronic device in the form of a thin layer, for example less than 1 micron thick.
The performance of OFET devices is principally based upon the charge carrier mobility of the semiconducting material and the current on/off ratio, so the ideal semiconductor should have a low conductivity in the off state, combined with a high charge carrier mobility (>1×10−3 cm2 V−1 s−1). In addition, it is important that the semiconducting material is relatively stable to oxidation i.e. it has a high ionisation potential, as oxidation leads to reduced device performance. Further requirements for the semiconducting material are a good processability, especially for large-scale production of thin layers and desired patterns, and high stability, film uniformity and integrity of the organic semiconductor layer.
For application in bulk heterojunction (BHJ) organic photovoltaic (OPV) cells, a semiconductor is required that have a low band-gap, enable improved light harvesting by the photoactive layer and can lead to higher cell efficiencies.
Further requirements for the semiconductor are a good processability, especially for large-scale production of thin layers and desired patterns, and high stability, film uniformity and integrity of the organic semiconductor layer.
In prior art conjugated polymers based upon the benzo[1,2-b:4,5-b′]dithiophene were reported for example in U.S. Pat. No. 7,524,922 B2. Primarily, the disclosed benzo[1,2-b:4,5-b′]dithiophene polymers were to be used as an organic semiconductor in transistor applications. However, specific co-polymer variations are attractive candidates for photovoltaic applications, specifically in BHJ OPV devices.
By the incorporation of the electron-donating benzo[1,2-b:4,5-b′]dithiophene unit and an electron-accepting unit into a co-polymer i.e. a “donor-acceptor” polymer, a reduction of the bandgap can be achieved, which enables improved light harvesting properties BHJ OPV devices. For example, Yang and co-workers have reported co-polymers of 4,8-dialkoxybenzo[1,2-b:4,5-b′]dithiophene with various electron-accepting units to yield optical bandgaps of 1.05-1.70 eV.[see J. Hou, M.-H. Park, S. Zhang, Y. Yao, L.-M. Chen, J.-H. Li and Y. Yang, Macromolecules, 2008, 41, 6012]. In comparison, they reported the homopolymer of 4,8-dialkoxybenzo[1,2-b:4,5-b′]dithiophene to have an optical bandgap of 2.13 eV. Further reports of co-polymers of 4,8-dialkoxybenzo[1,2-b:4,5-b′]dithiophene with thieno[3,4-b]thiophene carboxylate and thieno[3,4-b]thiophene ketone units respectively have been made and are summarized in Table 1 [see Y. Liang, Y. Wu, D. Feng, S.-T. Tsai, H.-J. Son, G. Li and L. Yu, J. Am. Chem. Soc., 2009, 131, 56 and J. Hou, H.-Y. Chen, S. Zhang, R. I. Chen, Y. Yang, Y. Wu and G. Li, J. Am. Chem. Soc., 2009, 131, 15586]. An additional report of a co-polymer of 4,8-dialkylbenzo[1,2-b:4,5-b′]dithiophene with a thieno[3,4-b]thiophene carboxylate unit has been made and is also summarized in Table 1 [see Y. Liang, D. Feng, Y. Wu, S.-T. Tsai, G. Li, C. Ray and L. Yu, J. Am. Chem. Soc., 2009, 131, 7792].
By the expansion of the electron-donating 4,8-dialkylbenzo[1,2-b:4,5-b′]dithiophene unit with the addition of two 3-alkylthiophene units to yield the novel 2,6-bis(3-alkyl-2-thienyl)-4,8-dialkylbenzo[1,2-b:4,5-b′]dithiophene unit, the copolymer's solubility and electronic properties can be further modified. Meanwhile, Ong and co-workers have claimed the application in transistor devices of the homopolymer, poly[2,6-bis(3-alkyl-2-thienyl)-4,8-dialkylbenzo[1,2-b:4,5-b′]dithiophene] [see EP 1 916 250 A1], but there are no reports of “donor-acceptor” co-polymers incorporating this unit nor their application in BHJ photovoltaic devices.
US 2010/0078074 A1 discloses a polymer comprising repeat units of the following formula as active material for photoelectric devices:
wherein R1, R2, R3 and R4 are alkyl or alkoxy groups with 1 to 18 C atoms, and Ar1 is a mono-, bi- or polycyclic N-containing heteroarylene with 1 to 5 N atoms. However, the alkyl substituents R3 and R4 at the thiophene rings pointing towards the heteroarylene group Ar1 can cause problems, especially in case R3 and R4 are longer alkyl chains and Ar1 is a bulky group that contains e.g. a benzene ring, due to steric hindrance which disturbs the planar structure of the polymer that is desired for good charge transport properties.
WO 2010/135701 A1 discloses random copolymers of first and second monomeric units, each comprising one benzodithiophene, one benzothidiazole and two thiophene rings, wherein in the first monomeric unit all thiophene rings are unsubstituted and in the second monomeric unit all thiophene rings are substituted, however, it does not disclose polymers as claimed in the present invention.
Therefore, there is still a need for OSC materials that do not have the drawbacks of prior art materials and are suitable for use in OFET and BHJ OPV devices.
The materials should be easy to synthesize, show good structural organization and film-forming properties, exhibit good electronic properties, especially a high charge carrier mobility, good processibilty, especially a high solubility in organic solvents, and high stability in air. For use in OPV cells, they should have a low band-gap, which enable improved light harvesting by the photoactive layer and can lead to higher cell efficiencies. For use in OFETs there is also a need for OSC materials that allow improved charge injection into the semiconducting layer from the source-drain electrodes.
It was an aim of the present invention to provide improved polymers for use as OSC materials especially in BHJ OPV devices, but also in OFET devices, which show the above-mentioned advantageous properties, and which do not show the above-mentioned disadvantages of prior art materials. Another aim of the invention was to extend the pool of OSC materials available to the expert. Other aims of the present invention are immediately evident to the expert from the following detailed description.
The inventors of the present invention have found that these aims can be achieved by providing the polymers as described hereinafter, which comprise benzodithiophene units as electron-donating units, thiophene units, and optionally comprise electron-accepting units like for example benzothioadiazole.